Lake freighters, or Lakers, are bulk carrier vessels that ply the Great Lakes. The most well-known is the SS Edmund Fitzgerald, the latest major vessel to be wrecked on the Lakes. These vessels are traditionally called boats, not ships. In the mid-20th century, 300 lakers worked the Lakes but by the early 21st century, there were fewer than 140 active lakers. Visiting ocean-going vessels are called "salties." Due to their additional beam, very large salties are never seen inland of the Saint Lawrence Seaway. Because the largest of the Soo Locks is larger than any Seaway lock, salties that can pass through the Seaway may travel anywhere in the Great Lakes. Because of their deeper draft, salties may accept partial loads on the Great Lakes, "topping off" when they have exited the Seaway. Similarly, the largest Lakers are confined to the Upper Lakes (Superior, Michigan, Huron, Erie) because they are too large to use the Seaway locks, beginning at the Welland Canal that bypasses the Niagara River. Lakers carry bulk cargoes of materials such as limestone, iron ore, grain, coal or salt from the mines and fields to the highly populated industrial areas down the lakes. The 63 commercial ports handled 173 million tons of cargo in 2006. Because of winter ice on the lakes, the navigation season is not usually year-round. The Soo Locks and Welland Canal close from mid-January to late March and most boats are laid up for those months for maintenance and repair. Crewmembers spend these months ashore.